1. Field of the Invention
The present invention relates to a cathode ray tube having a cold cathode electron gun.
2. Description of the Related Art
A cold cathode known as the Spindt type includes a conical emitter and a gate electrode having a gate aperture provided so as to surround the tip of the emitter. By applying a voltage ranging from several tens to 100 V, a strong electric field is generated at the tip of the conical emitter and electrons are emitted from the tip of the emitter.
Whereas electrons emitted from a hot cathode have an initial velocity corresponding to their thermal energy (i.e., not more than a few eV), electrons emitted from the cold cathode have an initial velocity of several tens to 100 eV, corresponding to the difference in applied voltage between the emitter and the gate electrode.
Further, since electrons are emitted not only from the tip of the emitter but also from minute projections formed on the surface of the emitter, some electrons are emitted at a certain angle with respect to a central axis of the conical emitter. The angle at which an electron is emitted with respect to the central axis of the conical emitter is referred to as an “emission angle”. Although the emission angle varies depending on the shape and the potential of the emitter and the gate electrode, it generally is known that the emission angle is about 30°. In contrast, in the hot cathode, electrons are emitted at an emission angle of 90°.
Therefore, in the cold cathode, an electron beam is emitted at an initial velocity ranging from several tens to 100 eV and at an emission angle of about 30°. The initial velocity of an electron beam in the cold cathode is several tens of times greater than that in the case of the hot cathode (in which an electron beam is emitted at an initial velocity of a few eV and at an emission angle of 90°). Therefore, when the cold cathode is used as a cathode of an electron gun in a cathode ray tube, an electron beam emitted from the emitter has a certain divergence angle when entering an electrostatic lens region of the electron gun. The electron beam having such a divergence angle cannot be made narrower easily by the electrostatic lens to be encountered later. As a result, a small beam spot cannot be formed on a phosphor screen, which causes a decrease in resolution of a display image.
As a solution to this problem, JP 9(1997)-283009 A discloses a method for reducing the divergence of an electron beam. In the following, this method will be described with reference to FIGS. 6 and 7. In FIGS. 6 and 7, 1a denotes conical emitters and 2 denotes a gate electrode having openings (gate apertures) surrounding the respective emitters 1a. 
In FIG. 6, the region where the emitters 1a are formed and the gate electrode 2 are circular, and a first focus electrode 13 is disposed on the outside of them so as to be coplanar with the gate electrode 2. To the first focus electrode 13, a potential lower than that of the gate electrode 2 is applied. By providing the first focus electrode 13 having a lower potential than that of the gate electrode 2 on the outside of the region where the emitters 1a are formed, a focus effect acts on an electron beam from the outside, thereby reducing the divergence of the electron beam.
On the other hand, in FIG. 7, the region where the emitters 1a are formed and the gate electrode 2 are in a ring shape, and a first focus electrode 13 and a second focus electrode 14 are disposed on the outside and the inside of them, respectively, so as to be coplanar with the gate electrode 2. To the first focus electrode 13 and the second focus electrode 14, a potential lower than that of the gate electrode 2 is applied. By providing the first focus electrode 13 and the second focus electrode 14, each having a lower potential than that of the gate electrode 2, on the outside and the inside of the region where the emitters 1a are formed, a convergence effect acts on an electron beam from the outside and the inside, thereby reducing the divergence of the electron beam.
However, in the case where the circular region with the emitters 1a shown in FIG. 6 has a greater diameter, and in the case where the ring-shaped region with the emitters 1a shown in FIG. 7 has a greater width in its radial direction, electron beams emitted from the emitters 1a in the vicinity of the first focus electrode 13 or second focus electrode 14 and those emitted from the emitter 1a apart from these electrodes are subjected to different degrees of convergence effect and thus have different degrees of divergence. That is, according to the technique shown in FIGS. 6 and 7, although the divergence of the electron beams emitted from the emitters 1a in the vicinity of the first focus electrode 13 or the second focus electrode 14 can be reduced, the divergence of the electron beams emitted from the emitter 1a apart from these electrodes cannot be reduced sufficiently.